Effects of whey protein and leucine supplementation on insulin resistance in non-obese insulin-resistant model rats.

OBJECTIVE: Whey protein (WP) has been reported to reduce body weight gain and improve glucose metabolism in obese individuals. This study aims to assess and compare the effects of WP and its hydrolysate-leucine (Leu) supplementation in non-obese, insulin-resistant (IR) rat models, particularly the effects on insulin sensitivity, lipid profile, and antioxidant activity.

METHODS: Wistar rats were fed a diet consisting of 38.5% fat for 12 wk and 51.3% fat for an additional 4 wk to establish non-obese IR rats. The IR rats were then switched to regular AIN-93 diet containing 0% WP, 5% WP, 15% WP or 1.6% Leu for 8 wk. The Leu content was the same in the 15% WP and 1.6% Leu groups based on high-performance liquid chromatography. The IR rats' body weight, fasting blood glucose, fasting insulin, and homeostasis model assessment-insulin resistance were measured before and after supplementation. An oral glucose tolerance test was performed after supplementation. Body composition, plasma concentrations of the lipids profile, and antioxidant index also were analyzed.

RESULTS: No significant difference was observed in body weight, energy intake, and fasting blood glucose in the non-obese IR rats at the end of the experiment. Compared with the 0% WP group, the fasting insulin and homeostasis model assessment-insulin resistance significantly decreased in the 15% WP and 1.6% Leu groups. Furthermore, the blood glucose area under the curve of the oral glucose tolerance test was significantly less in the 15% WP and 1.6% Leu groups. There were no differences in the lipids profile, except for the increase in the high-density lipoprotein cholesterol in the 15% WP and 1.6% Leu groups. For the antioxidant index, the 15% WP group had significantly increased plasma levels for total antioxidation capacity, superoxide dismutase, and glutathione, and a decreased malondialdehyde concentration. The 1.6% Leu group was shown to have the same effect as the 15% WP group, except for the glutathione.

CONCLUSION: Our findings demonstrate that the supplementation of WP and Leu may improve IR and antioxidant stress without resulting in changes in body weight and energy intake in non-obese IR rats.